1. Field of the Invention
Methods and apparatuses consistent with the present invention relate to audio coding and decoding, and more particularly, to coding and decoding an amplitude of partials in a parametric codec.
2. Description of the Related Art
A parametric codec is a combination of parametric coding for parsing and parametrizing an audio signal and parametric decoding for reconstructing a parameter to an audio signal. FIG. 1 is a flowchart of a related art parametric coding method. Parameters for audio components in each domain are extracted by performing three types of analysis, i.e., performing transient analysis in operation 101, performing sinusoidal analysis in operation 102, and performing noise analysis in operation 103.
The transient analysis deals with a dynamic audio change. The sinusoidal analysis deals with a deterministic audio change. The noise analysis deals with a stochastic or non-deterministic audio change. The extracted parameters are formatted into a bitstream in operation 104.
In related art parametric coding, the sinusoidal analysis involves analyzing a sinusoid of an input audio signal in order to generate partials and tracking generated partials. The partials are divided into continuation partials and birth partials by the tracking. As illustrated in FIG. 2, the continuation partials are related to partials of a previous frame and the birth partials are newly generated irrespective of the partials of the previous frame.
Related art parametric coding has more continuation partials than birth partials. Thus, a reduction in the number of bits for expressing the continuation partials and an improvement in the sound quality of the continuation partials exert a large influence on the reduction in the total number of bits and improvement in overall audio quality.
Referring to FIG. 3, in related art parametric coding, amplitude coding of continuation partials is performed by obtaining a quantized value Q_P of the amplitude of partials of a previous frame using a log scale method in operation 301 and a quantized value Q_C of the amplitude of partials of the current frame using a log scale method in operation 302. Next, a difference between the quantized value Q_P and the quantized value Q_C, i.e., D=Q_C−Q_P, is obtained in operation 303 and the obtained difference D is formatted into a bitstream in operation 304, thereby reducing the number of bits of continuation partials after being coded.
However, during amplitude coding of continuation partials, if the amplitude of the continuation partials gradually increases or decreases, a gradual amplitude variation cannot be expressed and only a large amplitude variation can be expressed, resulting in a step phenomenon in an amplitude variation as illustrated in FIG. 4.
FIG. 4 is a graph showing a relationship between the amplitude of continuation partials before being coded with respect to preset amplitude granularities (amp_granularity) and an amplitude variation with respect to a change of 1 in the quantized value Q_C. In FIG. 4, a horizontal axis indicates the amplitude of continuation partials before being coded and a vertical axis indicates an amplitude variation when the quantized value Q_C changes by 1.
Referring to FIG. 4, for an amp_granularity of 0, a step phenomenon does not occur in an amplitude variation. However, as the amp_granularity increases, the step phenomenon occurs to a large extent in the amplitude variation, causing degradation in reproduced sound quality.
Moreover, in related art amplitude coding of continuation partials, a difference between a quantized value of the amplitude of partials of a previous frame, which is obtained using a log scale method, and a quantized value of the amplitude of partials of the current frame, which is also obtained using the log scale method, i.e., the difference D in FIG. 3, is formatted into a bitstream without considering a frequency domain, causing a failure in efficiently reducing the number of bits of continuation partials. Furthermore, a large number of bits are used for expressing an amplitude variation in a small-amplitude portion that is not perceivable by human ears, resulting in a failure to efficiently reduce the number of bits in coding an audio signal.